For productions like Game of Thrones or The Hobbit, I assume prop and dress production is relatively easy -- While we have drawings and paintings, no one is alive who remembers what people wore in the middle ages, you can be a bit off in your representation and no one will care, for instance Ikea rugs were used for the nights watch cloaks.
For productions like Stranger Things however a good chunk of the target demographic remembers the eighties and remembers them pretty well. Props like the wired telephones and CRT TVs reminiscent of the eighties help to complete the feel for a more discerning audience.
How do productions acquire these props? Do they need to build fake replicas, do they go hunting for them at garage sales? Do studios just buy modern electronics/outfits/props and stores them for possible use twenty-some-odd-years down the line?